The present invention relates to sealing wires and sealing wire assemblies used in the packaging industry for fusing plastic film material and, more particularly, to a system for controlling the temperature of a sealing wire and to the attachment of the sealing wire in a sealing jaw.
In the plastic packaging industry, sealing heads having heated wires are frequently used to form seams in plastic film packages. Sealing heads are used to fuse sheets of plastic film to one another by melting the sheets of plastic film along specific areas, thereby forming seams.
Temperature control of the sealing wires is a major problem in sealing systems, yet temperature is an important factor in producing strong and effective seams. For instance, if a sealing wire is too cool, sealing cycles will be unnecessarily long and could provide weak seams. On the other hand, if a sealing wire is too hot, the sheets of film to be joined may be destroyed by the excessive temperature or the seals may have holes. Other problems occurring with excessively hot sealing wires include sheets of film sticking to the sealing head and gumming the sheets of film as well as causing hazardous smoke and fumes. The fused sheets of film may also leave residue on the sealing head or jaws. In the last case, the residue left on the sealing head can cause uneven temperatures along the sealing wire, leaving weaker seam sections.
It is a feature of the present invention to provide a new system for controlling the temperature of a sealing wire.
It is a further feature of the present invention to provide a method for controlling the temperature of the sealing wire of the present invention.
Another feature is to provide a new sealing head providing ease of replacement of the sealing wire.
According to the above features, from a broad aspect, the present invention provides an apparatus for sealing plastic film sheets together, comprising a first jaw having an elongated body with a first edge surface and a bore in the body having temperature access means. A second jaw has an elongated body with a second edge surface. A sealing wire is positioned on the first edge surface. The first jaw and the second jaw are displaceable to a closed position with respect to one another wherein the first edge surface of the first jaw and the second edge surface of the second jaw are opposed with the sealing wire therebetween such that sheets of plastic film positioned between the first and the second jaw in the closed position are sealed together by the sealing wire. A power supply is provide to heat the sealing wire. A temperature sensor device is received in the throughbore and positioned such that the temperature access means is visible by the temperature sensor device through the bore and such that the temperature sensor device is without contact with the sealing wire so as to measure a temperature of the sealing wire for controlling the power supply to maintain a substantially constant desired sealing wire temperature.
According to a further broad aspect of the present invention, there is provided a system for sealing sheets of plastic film together, comprising an apparatus having a first jaw, a second jaw, a sealing wire and a temperature sensor device. The first jaw and the second jaw are displaceable with respect to one another to a closed position, wherein the first jaw and the second jaw are opposed to one another with the sealing wire therebetween, such that sheets of plastic film positioned between the first and the second jaw in the closed position are sealed together by the sealing wire when heated during a sealing cycle. The temperature sensor device is positioned to measure a temperature of the sealing wire during the sealing cycle. A power supply is connected to the sealing wire to supply an electric current thereto for heating the sealing wire. A controller is connected to the power supply and to the temperature sensor device for receiving temperature signals representative of the sealing wire temperature during the sealing cycle. The controller is inputted with a desired temperature signal from an operator. The controller automatically adjusts the value of the electric current from the power supply to the sealing wire as a function of the temperature signals from the temperature sensor device and the desired temperature signal.
According to a still further broad aspect of the present invention, there is provided a method for sealing sheets of plastic film together at a desired sealing temperature, comprising the steps of i) measuring a temperature of a sealing wire of a sealing jaw being supplied with a known current pulse and without contact with the wire; and ii) heating the sealing wire to a desired sealing temperature to seal sheets of plastic film together by the sealing jaw by adjusting a second temperature of the sealing wire as a function of the known current pulse in relation to the first temperature and of a sensing of the second temperature, until the desired sealing temperature is obtained.